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11.
D. Charalambous P. C. Hendry M. Holmes G. G. Ihas P. V. E. McClintock L. Skrbek 《Journal of Low Temperature Physics》2006,145(1-4):107-124
Experimental approaches to the study of quantum turbulence (QT) in superfluid 4He in the low temperature limit, where the normal fluid density is effectively zero, are considered. A succinct general introduction covers liquid 4He, superfluidity, critical velocities for the onset of dissipation, quantized vortex lines and QT. The QT can be created mechanically by the oscillation of wires or grids above characteristic critical velocities. The interesting dynamics of the oscillating grid are discussed. It exhibits an enhanced effective mass due to backflow, as expected from classical hydrodynamics. It is found that the critical velocity attributable to the onset of QT production rises with increasing temperature. Oscillating objects like grids or wires create QT that is not well-characterized in terms of length scale, and the QT is not spatially homogeneous. The QT can be detected by the trapping of negative ions on vortex cores. Although the corresponding capture cross-section has not yet been measured, it is evidently very small, so that the technique cannot be expected to be a very sensitive one. In the future it is hoped to create well-characterized, homogeneous QT by means of a drawn grid. Improved sensitivity in the detection of QT is being sought through calorimetric techniques that monitor the temperature rise of the liquid caused by the decay of the vortex lines. 相似文献
12.
Nikias Chrysostomos L. Raghuveer Mysore R. Siegel John H. Fabian Miklos 《IEEE transactions on bio-medical engineering》1986,(4):435-452
The zero-delay wavenumber spectrum (ZDWS) estimation approach is proposed for the analysis of array ECG signals as an alternative to isopotential mapping (IM). The ZDWS approach is advantageous because the key parameters related to cardiac electrical activation can be easily identified in the frequency-wavenumber domain. The method explains the data as the sum of wide-band planewaves projected onto the array plane and provides an accurate estimate of their number and bearing. The slowness distribution of each of the planewaves is then obtained by estimating their temporal spectrum. Simulated data and experimental ECG data collected from a canine epicardial preparation during control and localized myocardial ischemic condition'are analyzed via the ZDWS and IM. It is demonstrated that the ZDWS methodology provides a means for an objective, robust, and repetitive characterization of the array data which can explain the modulation in the cardiac activation wavefront produced by an area of localized coronary artery occlusion. 相似文献
13.
Christodoulou Symeon Agathokleous Agathoklis Charalambous Bambos Adamou Aristides 《Water Resources Management》2010,24(13):3715-3730
Sustainable management of urban water distribution networks should include not only new methods for monitoring, repairing
or replacing aging infrastructure, but also (and more importantly) expanded methods for modelling deteriorating infrastructure,
for pro-actively assessing the risk of failure and for devising replace or repair strategies. The study presented herein describes a framework for proactive risk-based integrity monitoring of urban water
distribution networks and the results obtained from a case-study based on a 5-year data sample. A combination of artificial
neural network and statistical modelling techniques stemming from parametric and nonparametric survival analysis (Kaplan–Meier
survival curves with Epanechnikov’s kernel) are utilized in the investigation of identified risk factors and for estimation
of the forecasted time to failure metric. The data is stratified for different pipe groups for a more targeted analysis. 相似文献
14.
This paper is concerned with the application of a minimum principle derived for general nonlinear partially observable exponential-of-integral control problems, to solve linear-exponential-quadratic-Gaussian problems. This minimum principle is the stochastic analog of Pontryagin's minimum principle for deterministic systems. It consists of an information state equation, an adjoint process governed by a stochastic partial differential equation with terminal condition, and a Hamiltonian functional. Two methods are employed to obtain the optimal control law. The first method appeals to the well-known approach of completing the squares, by first determining the optimal control law that minimizes the Hamiltonian functional. The second method provides significant insight into relations with the HamiltoniJacobi approach associated with completely observable exponential-of-integral control problems. These methods of solution are particularly attractive because they do not assume a certainty equivalence principle, hence they can be used to solve nonlinear problems as well. 相似文献
15.
M.R. Raghuveer Chrysostomos L. Nikias John H. Siegel 《Computer methods and programs in biomedicine》1985,20(2):135-149
This paper presents a 3-dimensional (3-D) frequency-wavenumber spectrum estimation (FWSE) approach to the analysis of ECG signals. This approach treats the data as ‘wavefronts plus noise’ and provides a means of estimating key parameters associated with propagating wavefronts. A high resolution technique based on minimum variance representations of 3-D data fields (3-D CLS technique) is employed to obtain the FWSE. Computer simulation results that demonstrate the high resolution property of the technique when compared with the maximum-likelihood method of Capon are presented. Results of application of the technique to epicardial ECG data collected from a sensor array are also presented and discussed. 相似文献
16.
A new procedure is proposed for ARMA modeling of fourth-order cumulants and trispectrum estimation of non-Gaussian stationary random processes. The new procedure is applied to the identification of nonminimum phase systems for both phase and magnitude response estimation. It is demonstrated by means of comprehensive simulation examples that the ARMA approach exhibits improved performance over conventional trispectrum methods. ARMA model order selection criteria based on fourth-order cumulants are presented and their performance evaluated. The computational complexity of the ARMA and conventional trispectrum methods is also examined. The new procedure does not require knowledge of the non-Gaussian distribution.This work was supported by the Office of Naval Research under Contract No. ONR-N00014-86-K-0219. 相似文献
17.
BR Martin DR Compton SF Semus S Lin G Marciniak J Grzybowska A Charalambous A Makriyannis 《Canadian Metallurgical Quarterly》1993,46(2):295-301
One aspect of cannabinoid structure-activity relationships (SARs) that has not been thoroughly investigated is the aromatic (A) ring. Although halogenation of the side chain enhances potency, our recent observation that iodination of the A ring also enhanced activity was surprising. The purpose of this investigation was to establish the steric and electrostatic requirements at these sites of the cannabinoid molecule via molecular modeling, while determining pharmacological activity. Molecular modeling was performed using the Tripos molecular mechanics force field and the semiempirical quantum mechanical package AM1. The Ki values for novel cannabinoids were determined in a [3H]CP-55,940 binding assay and ED50 values generated from four different evaluations in a mouse model. The present studies underscore the increase in potency produced by a dimethylheptyl (DMH) side chain. Trifluoro substitutions on the pentyl side chain, or bromination of the DMH side chain, had little effect on the pharmacological activity. Any substitution at the C4 position of the aryl ring resulted in a loss of activity, which appears to be due to steric hindrances. Nitro, but not iodo, substitution at the C2 position essentially produces an inactive analog, and the drastic alteration of the electrostatic potential appears to be responsible. The altered pharmacological profile of the 2-iodo analog seems to be related to an alteration in the highest occupied molecular orbital because there is no alteration in the electron density map compared to delta 8-tetrahydrocannibinol. 相似文献
18.
Harry Charalambous Shikhar Krishn Jha Kent Harrison Christian Ryan Thomas Lay Thomas Tsakalakos 《Journal of the European Ceramic Society》2018,38(10):3689-3693
In conventional flash sintering, the current rises nonlinearly to a set current limit, accompanied by a spike in the power density. This sudden power spike may cause hot spot formation, in which current preferentially channels through a small area, causing localized melting while other areas remain unsintered. By using a controlled current ramp early on the sudden power spike can be avoided. In addition, by changing the ramp rate material properties such as porosity, grain size and conductivity can be tuned. 相似文献
19.
D. Morris C. J. Buckley G. R. Morrison A. G. Michette P. A. F. Anastasi M. T. Browne R. E. Burge P. S. Charalambous G. F. Foster J. R. Palmer P. J. Duke 《Scanning》1991,13(1):7-10
A scanning x-ray microscope has been built at the Daresbury synchrotron radiation source and used to obtain images at soft x-ray wavelengths (~2–4 nm). The focusing element of the microscope is a zone plate, fabricated by a contamination writing process developed at King's College. Zone plates with the finest zone widths, and hence theoretical resolutions, below 20 nm have been fabricated in this way. The contamination material is carbon, and as such is unsuitable for high-efficiency focusing. In order to increase the zone plate efficiency, a shadowing pattern transfer process has been developed and used to replicate contamination lines with 25 nm widths into more suitable materials. 相似文献
20.
Elaheh Taghaddos Harry Charalambous Thomas Tsakalakos Ahmed Safari 《Journal of the European Ceramic Society》2019,39(9):2882-2888
Lead-free, (BiNa0.88K0.08Li0.04)0.5Ti0.995 Mn0.015 O3 piezoceramic has been successfully densified by a novel electrical current applied technique known as flash sintering (FS) at 880 °C. The effect of alternating and direct current, current density limit and holding time on the densification, crystal structure, electromechanical and electrical properties have been investigated. The optimum flash condition was obtained with a 1 KHz alternating current, 100 V·cm?1 initial electric field and preset maximum current limit of 1.5 A·cm-2. The flash sintered specimen is characterized with finer grain size (10–15 μm), slightly higher electromechanical properties and higher symmetry butterfly shape strain hysteresis loop compared to conventional sintering. Under both sintering conditions uniform distribution of elements and pure rhombohedral structure were observed. Flash sintering also results in lower resistivity and more significant grain boundaries contributions in the conduction mechanism. 相似文献